An external cavity, surface-emitting, optically pumped semiconductor-laser (hereinafter referred to as an OPS-laser) can provide a viable alternative to a diode-pumped solid-state laser (DPSS laser) in certain laser applications. This is particularly true when the laser is required to provide continuous-wave (CW) output. An intracavity frequency-multiplied OPS-laser pumped by radiation from a diode-laser array can provide several watts (W) of harmonic-wavelength output. Such OPS lasers are described in detail in U.S. Pat. No. 6,097,742, assigned to the assignee of the present invention, the complete disclosure of which is hereby incorporated by reference.
In a basic form, such a laser includes a laser-resonator having at one end thereof a structure referred to by practitioners of the art as an “OPS-chip”. This chip includes a surface-emitting semiconductor gain-structure surmounting a mirror-structure. The mirror structure can be a multilayer semiconductor mirror, a multilayer dielectric mirror, or a dielectric layer enhanced metal mirror. The gain-structure comprises a plurality of active (quantum-well) layers periodically separated by spacer layers. The quantum-well layers provide very high optical-gain. The chip is mounted on a heat sink.
The composition of the active layers of the gain-structure determines the fundamental wavelength of the laser. The mirror-structure of the OPS-chip is made maximally reflective at the fundamental wavelength of the laser and usually provides one terminating mirror of the laser-resonator. The gain-structure is accordingly in the resonator. Either a mirror at an opposite end of the resonator or an intermediate mirror (if the resonator is folded thereby) can be made partially transparent to fundamental radiation, or maximally transparent to harmonic-wavelength radiation, for coupling output-radiation out of the resonator. Pump radiation (usually from a diode-laser array) is focused on the gain-structure from in front of the OPS-chip forming what is usually termed a “pump-spot” on the gain-structure
Providing that the gain-structure of an-OPS laser is located at one end of the resonator coupled with the fact that the gain-structure, being spatially periodic, does not suffer from the phenomenon referred to as spatial hole-burning, makes it ideal for providing single-mode output (TEM00). However, more power can be extracted from the laser if the resonator is slightly “detuned” to allow a few lateral modes to oscillate. This is typically accomplished by axially moving the opposite end mirror of the resonator to vary the length of the resonator, while monitoring the output power.
It has been found that variation of output power with varying resonator length is not always a smooth variation that permits accurate optimization. The variation is sometimes erratic and unstable which makes it difficult to determine if an optimum resonator length has been reached. It is believed, without being limited to a particular theory, that one reason for this is that as the mirror is moved, the nominal circulating beam position (mode-spot) on the gain structure can shift laterally and become misaligned with the pump-spot. Whatever the reason, it would be useful if such uncertain behavior could be minimized, if not altogether eliminated.